POSSIBLE CAUSES:
Painting a warm surface in direct sunlight.
Application of solvent-based paint over a damp or wet surface.
Moisture escaping through the exterior walls (less likely with water-based paint than with solvent-based).
Excess resin on areas on timber.
Underbound / powdery substrate beneath older paint coatings
SOLUTION:
If blisters originate at the substrate, try to remove the source of moisture.
Ensure any underlying chalky / powdery surfaces are fully stabilised.
If blisters do not go all the way down to the substrate, remove them by scraping, and identify and cure any underlying defects.
Remove as much resin as possible. Consider using a heat gun, and wipe away any excess resin prior to applying a knotting solution.
In extreme cases resinous knots may have to be removed and filled.
